Everything on the plant takes some energy to produce but leaves are not sucking up nutrients that would otherwise go to the buds. Photosynthesis converts carbon dioxide and water/nutrients using light energy into oxygen and food for the plant. Even if the leaves are not in direct light if there is light and the leaves are green photosynthesis will occur. Just not as efficient as the leaves in direct light.
I've posted these pictures before but as you can see I didn't trim a single leaf. The buds were very large and when harvested the bud that was shaded from the light was solid and well formed. This is under just a 600 watt HPS. As you can see the jungle of leaves did not take anything away from the buds. They grew nice and big fueled by the photosynthesis taking place converting carbon dioxide, water, and light energy into food for the plant.